The present invention relates to an HF-excited gas laser with at least one laser tube fabricated of quartz (SiO2) or a ceramic or glass material, and in particular to such a laser tube which is resistant to the degradation by secondary radiation.
One of the most common industrial lasers is the CO2 laser. which is used, for example, for material processing. Such lasers are used and marketed by Applicant""s assignee, Trumpf GmbH And Co. in connection with cutting and/or welding devices. Von Borstel et al U.S. Pat. No. 5,115,446 granted May 19, 1992, may be referred to for further details as to the construction of such a gas laser.
Generally, such CO2 lasers have a laser resonator, which, in order to reduce the space required, may consists of several laser tubes connected with each other by mirror units both as to flow dynamics and optically. On the outside of the laser tubes, there is a number of electrodes which excite the laser gas by high frequency discharges. The laser tubes preferably consist of quartz (SiO2 ) with a wall thickness of a few millimeters.
In the case of laser tubes of this type, and in particularly in the case of CO2 lasers, it is found that defects, such as melted areas, breaks or cracks, appear in the laser tubes after a large number of operating hours. Previously, it was not known what cause these breaks or cracks.
Even in the case of laser tubes which are fabricated of a ceramic material such as Al2O2, or of a glass material, after a certain number of operating hours the appearance of defects is observed. For example, in the case of laser tubes consisting of Al2O2 in CO2 lasers, it is observed that holes form after a certain operating time so that the laser tube is no longer gas-tight.
The object of the present invention is to provide a laser in which the service life of the laser tubes, which, particularly those of quartz (SiO2 ) or a ceramic or glass material, is prolonged in comparison with the service life of tubes of these materials made in accordance with the prior art.
A Hxe2x80x94F excited gas laser which solves this problem on the basis of its design in accordance with the present invention, has a laser tube of ceramic material containing a laser gas (2). The wall of the laser tube includes a chemical component substantially precluding the passage therethrough of secondary radiation generated by the laser gas and having a wavelength in a proscribed range.
The invention is based on the following concept of the mechanism which leads to the formation of defects in of the laser tube.
During the operation of the laser in the case of HF-excited lasers and particularly, CO2 lasers, it has been found that, in addition to the laser radiation of the desired UV wavelength, secondary radiation with a wave length in a range other than that of the desired laser radiation also appears in the lasing gas. For example, in the case of a CO2 laser which emits laser light with a wave length of around 10.6 xcexcm, there also appears radiation with wave lengths which extend from the visual range to the UV range to wave lengths which are clearly shorter than 250 nm.
This radiation, which is described herein as xe2x80x9csecondary radiationxe2x80x9d hereafter for the sake of simplicity, is not oriented parallel to the longitudinal axis of the laser tube, but can be at any angle to the longitudinal axis, so that, in particular, it strikes the laser tube and can interact with this material from which the tube is fabricated.
The interaction of this comparatively high-energy secondary radiation with the material of the laser tube causes changes in the structure of the laser tube material, which are described below as xe2x80x9cdefectsxe2x80x9d. In particular, in the case of quartz (SiO2), but also in the case of glasses and ceramics, defects and so-called color centers, such as Exe2x80x2 centers, are created, and these have an electric dipole moment.
The dipoles of these defects can interact with the HF excitation energy being introduced through the tube. In this case, the defects can absorb a part of the excitation energy. The laser tube is changed locally by the absorbed energy and, in some cases, even melted locally. The stresses arising from the local alteration of the material after a certain number of operating hoursxe2x80x94as a rule several thousand operating hoursxe2x80x94lead to defects, such as breaks, cracks and/or holes in the laser tube. The defects can appear not only in the area through which the excitation energy is introduced into the laser gas, but also at other points.
Therefore, in accordance with the present invention, the laser tube is treated by suitable measures in such a way that the secondary radiation does not cause disabling defects in the material of the laser tubexe2x80x94particularly quartzxe2x80x94or so that the defects are limited to a very small area of the wall which is not dangerous for the operation of the laser. Thus, the breaks, cracks, melted spots, or holes which result from the absorption of the HF excitation energy in the secondary radiation are avoided or materially limited in location.
These measures are alternatively or cumulatively:
applying on the inside surface of the laser tube, a layer which is essentially impermeable to the secondary radiation and/or which reflects the secondary radiation (back) into the laser gas;
doping the material of the laser tube with foreign atoms or molecules in such a way that the secondary radiation is absorbed by the doping material in a manner which is xe2x80x9charmlessxe2x80x9d to the laser tube;
providing an additional inner tube, which absorbs or reflects the secondary radiation.
The layer provided on the inside of the laser tube in this case can be a vapor-deposited layer applied by a PVD or CVD process or a sol-gel process. The sol-gel process has the advantage in that it provides a simple way to coat the inside of the laser tube.
Examples of suitable layers are (1) an absorbing layer consisting of titanium oxide, cerium oxide, strontium oxide, or zirconium oxide, or a mixture of these oxides, or (2) a reflective layer and, in particular, a dichroic mirror layer of the type known from optics.
An additionally provided inner tube can form a composite structure with the actual outer laser tube. The additional tube in this case can absorb and/or reflect the secondary radiation.
The doping of the inner tube can be performed in very different ways and also act in very different ways. The doping material can reduce the depth of penetration of the UV radiation into the laser tube to such a small distance that the dipoles which interact with the HF excitation energy are formed only in a small border area of the laser tube, and not in the xe2x80x9cmain partxe2x80x9d of the laser tube. Furthermore, the secondary radiation can be absorbed preferentially by the doping material instead of by the basic material of the laser tube. This can be done in such a way that there are formed no defects with a dipole moment to interact with the HF excitation energy and absorb it.
Suprisingly it has been found that, in the case of quartz as the basic material for the laser tube, nearly all defects leading to damage of the material of the laser tube are avoided if the doping material or the coating is chosen so that only UV light with a wave length of less that 250 nm is absorbed.
It has been shown that, in the case of other materials, the dangerous radiation which causes the defects with a dipole moment which interact with the HF excitation energy, can be in another wave length range, and, in a given case even in the visual range.
TiO2 or titanium is a possible doping material in the case of a CO2 laser and a laser tube consisting of quartz which absorbs UV radiation with a wave length shorter than 250 nm. Of course, other doping material also can be used instead of titanium or in addition to titanium.
The concentration of titanium as a doping material in quartz in this case is essentially higher than the concentration in which titanium typically is present as an impurity in commercially obtainable (natural or synthetic) quartz materials. In particular, the concentration is greater than 150 ppm and preferably lies in the range of 500 ppm and more.
In this case, the doping materials may be present in the required high concentration homogeneouslyxe2x80x94for example, by addition to the SiO2 meltxe2x80x94or they may be incorporated only in a boundary layer adjacent the inner surface of the laser tube. Processes by which such a concentration in a boundary layer can be created are, for example, surface diffusion processes or ion exchange processes.
Of course, the measures in accordance with the invention are applicable to all kinds of laser gases, but they are particularly beneficial for CO2 lasers using laser tubes made of quartz (SiO2)
The measures in accordance with the invention can be used in the case of inductively or capacitatively excited gas lasers. However, they are particularly effective in the case of lasers in which at least one or both of the capacitatively exciting electrodes are located outwardly of the laser tube, so that the entire excitation energy has to go through the laser tube. In this case it is possible to use the preventive formation of the invention only in the area of the laser tube through which the HF energy is introduced.